*** NO NEED TO MERGE WITH 4.07 ***
    (but it would do no harm)

* Improve an error message when overlapping instance
  declarations are present.  Carl Witty reported this
  infelicitous message.  The problem arises for this code:

	class Foo a
	class (Foo a) => Bar a
	
	data Dat a = Dat
	
	instance Foo (Dat a)
	instance Foo (Dat Integer)
	
	instance Bar (Dat a)

  The instance decl for Bar should say

	instance Foo (Dat a) => Bar (Dat a) 

  because the overlapping instance decls for Foo can't
  be resolved (or at least might vary depending on how
  a is instantiated).